1. Field of the Invention
This invention relates to the field of explosive indicators. More particularly this invention relates to frangible circuits. In still greater particularity the invention relates to a conductor which is plated on a frangible substrate and is fractured by shock waves from an explosive force but is protected from external mechanical shocks by a resilient potting compound.
2. Description of the Prior Art
Devices for making or breaking electrical circuits have been used for a variety of purposes. These circuits have been used to signal other devices upon the occurrence of certain events. One such system uses the breaking of a circuit affixed to a window to trigger an alarm. The breaking of the window is the event with the concurrent breaking of the circuit providing the signal to trigger the alarm.
A frangible printed circuit which may be broken by a bending force is shown in U.S. Pat. No. 3,072,500 issued to William L. Berlinghof on Jan. 8, 1963. this device provides for circuit breakage due to mechanical or bending force and, while suitable for this purpose, it would be less than satisfactory where only explosive signals are to be recognized.
One recent application of devices for making or breaking electrical circuits has been their use as a part of a vehicle collision detecting apparatus. Such a device is illustrated in U.S. Pat. No. 3,905,015 issued to Fumiyiki Inose on Sept. 9, 1975. In that device, a metal film is deposited on a glass substrate by evaporative deposition. The element is attached to a vehicle at a point where deformation is likely to occur. Lead wires, attached to the conducting metal film, convey a signal to a detecting circuit and actuate an airbag which is used for protection of the vehicle operator. While it is satisfactory for its intended purpose, the device is extremely frangible and, as such, is less satisfactory for use in situations where it is undesired, or even mandatory, that mechanical shock not break the circuit. Such would be the case if it is required to have the device recognize only an explosive shock.
Prior devices for monitoring an explosive event have used explosive force to bend metallic electrical contacts from an open circuit to a closed circuit configuration. The reliability of these devices is subject to question since the magnitude and direction of the explosive force is not completely predictable. In addition, switches of this type are relatively costly. Another disadvantage of the explosive switch is the relatively long activation time which may be on the order of 20 milliseconds. The present invention provides a reliable, cheap device which can perform the switching function in 100 microseconds.